1. Field of the Invention
The present invention relates to electric power steering apparatuses for automotive vehicles which provide output power of an electric motor to the vehicle steering system to reduce a steering effort to be manually applied by a vehicle driver. More particularly, the invention relates to an electric power steering apparatus which is capable of determining an actual winding resistance value of the electric assisting motor through arithmetic operations.
2. Description of the Related Art
Electric power steering apparatuses are widely used today in automotive vehicles. Improved electric power steering apparatuses have been known, which are arranged to constantly detect a rotating speed of the electric assisting motor and execute so-called "unload control" to decrease a target motor current when the detected rotating speed of the motor is below a predetermined value, (i.e., the motor is in a virtually stationary or nonrotating condition) and a current actually flowing through the motor is over a predetermined value, so as to avoid a waste of the current flowing through the motor.
Whereas a rotating speed sensor, typically in the form of a tacho-generator, is generally used to detect the motor rotating speed (M.sub.V), some of the electric power steering apparatuses, rather than using such a sensor, determine the motor rotating speed (M.sub.V) from following Equation (1) using a winding resistance value of the motor (R.sub.M), current actually flowing through the motor (I.sub.M) and voltage supplied to the motor (V.sub.M): EQU M.sub.V =V.sub.M -R.sub.M.times.I.sub.M Equation (1)
However, because the winding resistance value R.sub.M applied to Equation (1) is a fixed value as specified in the specifications of the motor, the motor rotating speed M.sub.V often can not be calculated accurately from Equation (1) if an actual winding resistance value R.sub.M of the motor in operation substantially differs from the specified fixed value. Thus, where the motor rotating speed M.sub.V calculated on the basis of the fixed winding resistance value R.sub.M is used for the unload control, the assisting motor would rotate at different speeds than those actually required, with the result that proper unload control is not achieved.